feat(config): support XDG_DATA_HOME on Windows and macOS

When XDG_DATA_HOME is explicitly set, dtvem will now use
$XDG_DATA_HOME/dtvem as the root directory on Windows and macOS,
matching the existing Linux behavior. This is opt-in only - when
XDG_DATA_HOME is not set, the default ~/.dtvem is still used.

Closes #202

Author: CalvinAllen

src/internal/config/paths.go

@@ -54,16 +54,18 @@ func initPaths() *Paths {
 //   - Otherwise uses ~/.local/share/dtvem (XDG default)
 //   - This is the standard location for user-specific data files on Linux
 //
-// macOS: Uses ~/.dtvem
+// macOS: Uses ~/.dtvem by default
 //   - macOS has its own conventions (~/Library/Application Support) but many CLI tools
 //     use dotfiles in home directory for better discoverability and Unix compatibility
 //   - ~/.dtvem is more familiar to users coming from tools like nvm, pyenv, rbenv
+//   - Optionally respects $XDG_DATA_HOME if set (for users who prefer XDG consistency)
 //
-// Windows: Uses %USERPROFILE%\.dtvem
+// Windows: Uses %USERPROFILE%\.dtvem by default
 //   - Alternatives considered: %LOCALAPPDATA% (C:\Users\<user>\AppData\Local)
 //   - Chose home directory for consistency with macOS/Linux and better visibility
 //   - Users expect CLI tool configs in their home directory
 //   - Easier to locate and backup than buried in AppData
+//   - Optionally respects $XDG_DATA_HOME if set (for users who prefer XDG consistency)
 //
 // Override: DTVEM_ROOT environment variable overrides all platform defaults
 func getRootDir() string {
@@ -79,12 +81,17 @@ func getRootDir() string {
 		return ".dtvem"
 	}
 
-	// On Linux, respect XDG Base Directory specification
+	// On Linux, always respect XDG Base Directory specification
 	if runtime.GOOS == constants.OSLinux {
 		return getXDGDataPath(home)
 	}
 
-	// On macOS and Windows, use ~/.dtvem
+	// On macOS and Windows, use XDG_DATA_HOME if explicitly set (opt-in)
+	if xdgDataHome := os.Getenv("XDG_DATA_HOME"); xdgDataHome != "" {
+		return filepath.Join(xdgDataHome, "dtvem")
+	}
+
+	// Default for macOS and Windows: ~/.dtvem
 	return filepath.Join(home, ".dtvem")
 }
 

src/internal/config/paths_test.go

@@ -384,8 +384,8 @@ func TestGetRootDir_XDGOnLinux(t *testing.T) {
 	}
 }
 
-func TestGetRootDir_NonLinux(t *testing.T) {
-	// On non-Linux platforms, verify that ~/.dtvem is used regardless of XDG
+func TestGetRootDir_NonLinux_WithXDG(t *testing.T) {
+	// On non-Linux platforms, verify that XDG_DATA_HOME is respected when set
 	if runtime.GOOS == constants.OSLinux {
 		t.Skip("This test only runs on non-Linux platforms")
 	}
@@ -409,15 +409,53 @@ func TestGetRootDir_NonLinux(t *testing.T) {
 
 	// Clear DTVEM_ROOT and set XDG_DATA_HOME
 	_ = os.Unsetenv("DTVEM_ROOT")
-	_ = os.Setenv("XDG_DATA_HOME", "/should/be/ignored")
+	customXDG := "/custom/xdg/data"
+	_ = os.Setenv("XDG_DATA_HOME", customXDG)
+	resetPathsForTesting()
+
+	result := getRootDir()
+	expected := filepath.Join(customXDG, "dtvem")
+
+	if result != expected {
+		t.Errorf("getRootDir() on %s should use XDG_DATA_HOME when set, got %q, want %q",
+			runtime.GOOS, result, expected)
+	}
+}
+
+func TestGetRootDir_NonLinux_WithoutXDG(t *testing.T) {
+	// On non-Linux platforms, verify that ~/.dtvem is used when XDG_DATA_HOME is not set
+	if runtime.GOOS == constants.OSLinux {
+		t.Skip("This test only runs on non-Linux platforms")
+	}
+
+	// Save original environment
+	originalRoot := os.Getenv("DTVEM_ROOT")
+	originalXDG := os.Getenv("XDG_DATA_HOME")
+	defer func() {
+		if originalRoot != "" {
+			_ = os.Setenv("DTVEM_ROOT", originalRoot)
+		} else {
+			_ = os.Unsetenv("DTVEM_ROOT")
+		}
+		if originalXDG != "" {
+			_ = os.Setenv("XDG_DATA_HOME", originalXDG)
+		} else {
+			_ = os.Unsetenv("XDG_DATA_HOME")
+		}
+		resetPathsForTesting()
+	}()
+
+	// Clear both DTVEM_ROOT and XDG_DATA_HOME
+	_ = os.Unsetenv("DTVEM_ROOT")
+	_ = os.Unsetenv("XDG_DATA_HOME")
 	resetPathsForTesting()
 
 	result := getRootDir()
 	home, _ := os.UserHomeDir()
 	expected := filepath.Join(home, ".dtvem")
 
 	if result != expected {
-		t.Errorf("getRootDir() on %s should ignore XDG_DATA_HOME, got %q, want %q",
+		t.Errorf("getRootDir() on %s without XDG_DATA_HOME should use ~/.dtvem, got %q, want %q",
 			runtime.GOOS, result, expected)
 	}
 }